The 18th running of what has become America's most famous long-distance mountain race commences Saturday at 6:30 am when about 1,800 riders will roll away from the Leadville 100 start line. Laying in wait for this giant fat-tire peloton is a 100-mile out-and-back ride through the Colorado Rockies that will take anywhere from six to 12 hours.

But time and distance are just a fraction of the Leadville equation. It is the altitude that separates this race from its brethren. Leadville proper is located at a lung burning 10,152 feet, and things go up from there. Peak race elevation is at the summit of the infamous Columbine Mine climb, where racers will struggle to find adequate oxygen at 12,550 feet.